On December 26, 1985, Strieber reportedly had an experience in which he thought he was abducted from his cabin in upstate New York by non-human beings of some kind. He wrote about these experiences in his first non-fiction book, Communion (1987). Communion is generally interpreted as a claim of alien abduction, but Strieber says that he draws no firm conclusions about the nature or source of his experience. He refers to the beings as "the visitors," a name chosen to be as neutral as possible, and leaves open the possibility that they are not extraterrestrials and even that they exist only in his mind. He has repeatedly expressed his frustration with what he feels are fantastic claims incorrectly attributed to him.

Communion was a Number 1 New York Times bestseller in the Non-Fiction category. Strieber went on to write three more autobiographical books about his experiences with the visitors, Transformation (1988), Breakthrough (1995), and The Secret School (1996). Each was commercially less successful than the last, with Transformation the only other title besides Communion to make the New York Times bestseller list.

http://en.wikipedia.org/wiki/Whitley_Strieber

Strieber was already a well-known author when he was vacationing with his wife and son over the Christmas holidays in 1985. His strange abduction would take place in an isolated cabin in the northern part of New York state. The lonely setting of the cabin caused him to have concerns for this family's safety, and he had only recently had a cutting edge security system installed.
Creature in the Bedroom:
Strieber had activated his system at about 11:00 PM on December 26, and his family began to retire for the evening. A few hours later, he heard a strange sound, which woke him from sleep. Thinking that he might have a burglar who had set off the alarm system, he went to check it out. As he did, he was shocked to see a creature standing in his bedroom.
Seeking Help:
The next thing that Strieber knew, he was sitting in the woods that encircled his cabin. He was at a loss to explain what had happened, and how he had gotten from the bedroom to the woods. His memories were lost, and he eventually sought the help of Dr. Donald F. Klein. Klein would perform regressive hypnosis on Strieber in an attempt to recover the lost time. The sessions would be helpful, and enable him to recover many of the details of what had really happened on the night of December, 26, 1985.

http://ufos.about.com/od/aliensalienabduction/p/strieber.htm

Stephen King horror stories dont offend me, but as far as I know he never claimed 'Carrie' was non-fiction...